Sediment Geochemistry and Benthic Foraminiferal Response to Fish Farming after Conversion from a Red Laver (Seaweed) Farm

To understand the effects of biodeposits discharged from a fish farm (which was previously a red laver farm) on geochemistry of the sediment and benthic ecology, trace metals and element analyses, Pb-210 dating, and benthic foraminiferal analyses were conducted on sediment cores collected below the fish farm cages. Sediment core sampling was also carried out at a control plot (160 m away) to understand the spreading of biodeposits. The geochemical effect of fish farming is pronounced. There was an increase in silt content and sedimentation fluxes of Zn-EX (Zn in excess fractions), Cu-EX, As-EX, and TPEX (total phosphorus in excess fractions) after red laver farming. The silt content increased from 45.01% to 50.07%, with a sediment accumulation rate of 0.61 cmly, on average. TPEX, with 0.15% 0.06% on average, showed that the fish farming effects increased by 0.22% it 0.07%. Zn-EX, Cu-EX, Cd-EX, and As-EX showed a positive relationship with TPEX increase: 19.92 +/- 1.79, 10.23 +/- 3.99, 0.57 +/- 0.19, and 0.92 +/- 0.04 mu g/cm(2)/y, respectively. The high sedimentation fluxes of Zn-EX and As-EX in the control plot may signify an expansion of influence of the fish farm by tidal currents. Seven benthic foraminifera appeared during the fish and red laver farming periods, with the dominant species being Cribroelphidium excauatum; however, they were characterized by a high similarity index of 88.1% and very low frequency of abundance, indicating had habitat conditions. Among them, the C. excaualum, Ammonia beccarii Ammonia ketienziensis Cribroelphidium,subarcticum assemblage ranged from 21 cm depth to the uppermost layer, characterized by increases in A. beccarii. This increase may have been caused by the accumulation of organic matter discharged from the fish farm after red laver farming, rather than from an increase in the trace metals. A. beccarii may he a species tolerant to fish farming.